Oh, the stuff at costco doesnt have the bag inside and stays fresh forever. I never ever buy the stuff in the bags because that stuff goes bad fast. When you get it home take it out of the bag, put a 1/2 paper towel (a whole one of the selectasize is perfect) on the bottom of the clamshell, then put the lettuce back in and put the other half of the paper towel on top. Pick any wilty leaves out right away and everything else will stay fresh longer.
KEEP 1-2 of the clamshells. During the times of year when lettuce is cheaper in the head these clamshells with paper towels keep the lettuce fresher longer than anything else I have ever tried. I keep 2 around for my CSA lettuce in the summer.